Derrick is the nerd of the McReary Family. In earlier comics it is shown that he apparently has an obsession with math, being comparable to his older brother Gerry's obsession with football. In the first comic of the series, Packie describes Derrick as a "total nerd and math geek".
As the series goes on (more specifically from 2015), Derrick is not really shown to be obsessed with just math as much anymore. He is now shown as a typical nerd that is very intelligent. While not being as smart as his two oldestbrothers, Derrick can be considered as the "brains" of the family.
Derrick's personality is that of a stereotypic nerd. He is smart with facts, and most of the things he says usually have the word "math" in his lines. Sometimes he even exclaims random facts.
While being an optimist like the rest of his family, Derrick also has a rude side to his personality, and insults his fellow siblings (see Trivia), but overall, he is still a kind and humorous character.
Derrick wears a buttoned collar shirt with plain pants and shoes. His hair is shorter than Packie's. To match his nerdy personality, he also wears glasses. From 2015 on, when Derrick is seen facing the reader in the comics, his glasses appear to be taped together, further going with his nerdy lifestyle.
Derrick has shown to be rude to his siblings twice in the series. The first time was in the comic Yearning to be Young, where in the future set in 2033, he gets angry with Packie acting childish and tells him to "grow up already". The second time was in The McRearys' Halloween Special!, where he argues with Francis about his Halloween costume, saying he doesn't even look like a killer hockey player (as the only thing he has is a mask and fake knife). Packie comments on this, saying "Wow, give Derrick a Twix and he gives you an ugly attitude!".
Apparently, as mentioned in "Yearning to be Young", Derrick used to be obsessed with geography rather than math in 2012, until "Russia made him angry".
However this is contradicted in SRA (which is set in 2012), where it is shown that he is still into math.